1. Field of the Invention
The invention relates to a passive component and more particularly to a thin film resistor structure and a method for fabricating the thin film resistor structure.
2. Description of the Related Art
Some essential elements for printed circuit boards (PCB), are copper foil wiring and passive components such as resistors. For conventional PCB fabrication, copper foil wiring is formed by forming a copper clad laminate (CCL), followed by a development, an etching, and a stripping process (hereinafter referring to as a DES process). Thereafter, discrete passive components may be mounted on the PCB by a surface mount technology (SMT) process. However, with more and more passive components being required on a PCB due to increased functions and miniaturization of electronic products, the area for devices on a PCB are becoming increasingly limited. In order to address the limitation, a major technological approach used, is to reduce the size of the passive components. However, it is extremely difficult to reduce the size of passive components to be smaller than the physiologic limits of vision in physiographic observation, like the 0201-type resistor, with the aforementioned processes.
In order to address the difficulty, planar embedded/buried resistors were developed in the 80's, to reduce the size of passive components on a PCB. Currently, the most popular embedded resistors are classified into thick film-type resistors and thin film-type resistors, in which thick film-type resistors have a thickness of more than 10 μm and thin film-type resistors have a thickness of less than 2 μm. Moreover, thick film resistors can be further classified into lower temperature co-fired ceramic (LTCC)-type resistors and polymer thick film (PTF)-type resistors. Thick film resistors have advantages of broad resistance range and low fabrication cost. However, thick film resistors have poor resistance tolerance. Specifically, for LTCC-type resistors, drawbacks include high processing temperatures and poor polymer substrate compatibility and for PTF-type resistors, drawbacks include a high temperature coefficient of resistance (TCR) and poor thermal stability. As such, applications for thick film resistors are limited. Conversely, thin film resistors have advantages of good polymer substrate compatibility, thermal stability and resistance tolerance when compared to thick film resistors, by employing a metal foil substrate. However, due to the constraint of low electric resistivity, applications for alloy thin film resistors are also limited. The commercially reachable resistance range of the alloy thin film resistors are too much low (i.e. ≦250Ω/) to meet the predominant resistance range requirements of most devices (i.e. 10000Ω/).
Accordingly, thin film resistors with high resistivity are needed to advance application of embedded resistors along with technological trends. Additionally, low TCR (e.g. <200 ppm/° C.) characteristics must not be sacrificed while achieving high resistivity, to prevent reduction of thermal stability.